Demands for liquid crystals as they are represented by TV screens and computer displays know no limits. Most of currently used liquid crystals are of nematic liquid crystal phase and account for 99% of their total. The rest 1% of them are those of chiral smectic C-type liquid crystal phase which are called ferroelectric liquid crystals and used in components such as finders for video cameras that require high speed responsiveness.
By the way, the currently used nematic liquid crystals leave much to be desired in that they are slow in switching speed, get blurred and have a preceding image left for a moment. In order to solve these problems, vigorous research and development works are underway for a next-generation liquid crystal having high speed responsiveness and enhanced functionalities.
A ferroelectric liquid crystal is about 1000 times as high in switching speed as a nematic liquid crystal, wider in the angular field of view and more power-saving. Having also a memory function, it is expected as a next-generation liquid crystal.
Further, an antiferroelectric liquid crystal in addition to having properties of a ferroelectric crystal has a capability of gray display and is much expected as a next-generation liquid crystal. An antiferroelectric liquid crystal has the problem, however, that it is hard to synthesize.
Let us explain the problems of these conventional liquid crystal in greater detail below.
FIG. 9 is a diagram illustrating the principles of operation of a nematic type liquid crystal display device currently in use. The liquid crystal display devices most widely used at present make use of a liquid crystal phase, called the nematic phase, as shown in FIG. 9 in which liquid crystal molecules are arrayed in low order. Applying an electric field to the liquid crystal phase in a display device between a pair of electrodes causes elongate rod-like liquid crystal molecules to move changing their state from one in which they are oriented parallel to the electrodes to one in which they are oriented perpendicular to the electrodes. Light switching in the device can thus be effected utilizing such movement of the molecules and their reversed movement. The liquid crystal phase in this case in which liquid crystal molecules are simply rod-like is easy in molecular design and also easy to synthesize and hence can be applied to a variety of liquid crystal molecular species. However, reorienting these molecules requires large energy, namely a large voltage, which in turn also reduces the switching speed of the device to as low as several to several tens milliseconds.
Mention is next made of a ferroelectric liquid crystal device. In a liquid crystal phase called the smectic C-phase, liquid crystal molecules are arranged in layers and molecules in each layer have their major axes each inclined at a fixed angle to a normal to the layer. In a liquid crystal phase called the chiral smectic C phase, a chiral group is introduced to a terminal group of the smectic C-type liquid phase molecule and the chiral group has a permanent dipole added thereto. It is the chiral smectic C phase which a ferroelectric liquid crystal display device uses.
FIG. 10 is a diagram illustrating the principles of operation of a ferroelectric liquid crystal display device. As shown in FIG. 10(a), a ferroelectric liquid crystal display device uses the chiral smectic C phase. Applying an electric filed to this liquid crystal phase allows light to be switched. As shown in FIG. 10(b), in the chiral smectic C phase, liquid crystal molecules are formed in layers and molecules in each layer have their major axes each inclined at a fixed angle to a normal to the layer. Each of these liquid crystal molecules has a permanent dipole at a chiral group thereof, and the orientation of its spontaneous polarization is rotated according to the direction in which an electric filed is applied. For example, FIG. 10(b) shows at A′ that an electric field is applied directed from the back side of the sheet of paper towards its front side (as indicated by the large arrow in the Figure). Then, liquid crystal molecules in each layer are rotated about their respective axes normal to the layer with their spontaneous polarization oriented towards the back side of the sheet of paper (as indicated by the small arrows in the Figure). It is shown also at A that an electric field is applied directed from the front side of the sheet of paper towards its back side (as indicated by the large arrow in the Figure). Then, liquid crystal molecules in each layer are rotated about their respective axes normal to the layer with their spontaneous polarizations oriented towards the front side of the sheet of paper (as indicated by the small arrows in the Figure). The orientation of a spontaneous polarization can thus be inverted by inverting the direction in which an electric field is applied, and this can be used to switch or selectively transmit and block a ray of light. Also, with this inversion effected by the rotation of liquid crystal molecules about axes normal to their layer as mentioned above, the device can be driven rapidly and at low voltage, giving rise to a switching speed about 1000 times higher than can be achieved with the nematic phase. Further, since the spontaneous polarization once oriented in a given sign remains so oriented even if the electric field is removed, the device is provided with a memory function.
Mention is next made of an antiferroelectric display device. In a liquid crystal phase called the smectic CA-phase, liquid crystal molecules are arranged in layers, molecules in each layer have their major axes each inclined at a fixed angle to a normal to the layer and they are so inclined in an opposite direction every other layer. In a liquid crystal phase called the chiral smectic CA phase, a chiral group is introduced to a terminal group of a smectic CA-type liquid phase molecule and the chiral group has a permanent dipole added thereto. It is the chiral smectic CA phase which an antiferroelectric liquid crystal display device uses.
FIG. 11 is a diagram illustrating the principles of operation of an antiferroelectric liquid crystal display device. An antiferroelectric liquid crystal display device uses the chiral smectic CA phase, and applying an electric filed to the liquid crystal phase allows light to be switched. As shown in the Figure, in the chiral smectic CA phase, while it is the same as in the chiral smectic C phase shown in FIG. 10 that liquid crystal molecules are formed in layers, molecules in each layer have their major axes each inclined at a fixed angle to a normal to the layer and the orientation of their spontaneous polarizations are rotated according to the direction is which an electric filed is applied, there is a difference in the state they take when the electric field is removed. Thus, in the state that there is effectively no electric field applied as shown at A±, liquid crystal molecules in one layer and those in a next are spontaneously polarized in mutually opposite directions and with these spontaneous polarizations canceled by one another such molecules of the entire body over all the layers as a whole takes an antiferroelectric state. It is seen therefore that in addition to having a high switching speed and a memory function as does a ferroelectric liquid crystal display device, an antiferroelectric liquid crystal display device has three stable states, permitting not only black and white displays but also a halftone display (gray display) to be produced. And, there have also been discovered antiferroelectric liquid crystal materials which allow a gray display to be continuously varied in gray level.
Although antiferroelectric liquid crystals are thus expected to be the most promising for next-generation liquid crystal device materials, there exists the problem that they are hard to manufacture.
To wit, while an antiferroelectric liquid crystal has so far been sought to be made in the belief that a group having a permanent dipole of F or CF3 must be added to a terminal alkyl chain of a smectic CA liquid crystal molecule, it has been found to be difficult in terms of synthetic techniques to add fluorine species to the smectic CA liquid crystal molecule. See: Shin Jikken Kagaku Koza (Lectures of New Experimental Chemistry) Maruzen, 14 (Synthesis and Reactions of Organic Compounds) [1], pp 308–331; Gendai Yuuki Kagaku (Modern Organic Chemistry) 3rd Ed., authored by K. P. C. Vollhards, N. E. Schore, Kagaku Dojin (chemical coterie), Chapter 6 (Nature & Reactions of Haloalkanes), pp. 242–249 (Effects of Nucleophilicity in SN2 Reactions). Even if one can successfully be synthesized, it often loses properties as the smectic CA liquid crystal. When an antiferroelectric liquid crystal molecule is sought to be synthesized, an optical isomer thereof is likely to be produced at the same time, which makes it difficult to obtain the antiferroelectric liquid crystal that is 100% optically pure.
Furthermore, now that it has not yet been theoretically elucidated how the molecular assembling state that brings about the antiferroelectric state is formed (see: Kagaku Sosetsu (The Elements of Chemistry) No. 22, (Chemistry of Liquid Crystals) compiled by the Chemical Society of Japan, Society Publishing Center, pp 111–126, Chapter 9 (the Appearance of Ferroelectric, Antiferroelectric and Ferrielectric Phases: by Hideo Takezoe in charge (Fundamentals & New Developments of Research of Liquid Crystal Materials), compiled by (Society of Yong Researchers of Liquid Crystals), Chapter 5: pp 65–102 (Phase Transformation and Optical Texture of Liquid Crystal): by Youichi Takanishi in charge, Chapter 14: pp 229–237 (Antiferroelectric Liquid Crystal Materials): by Yoshio Takano & Hiroyuki Nohira in charge; Handbook of Liquid Crystals edited by D. Demus, J. W. Goodby, G. W. Gray, H. W. Spiess, V. Vill, Vol. 2B, Wiley-VCH Publishing Co. (1998, New York) Chapter 3: Antiferroelectric Liquid Crystals: K. Miyachi, A. Fukuda pp. 665–691; Handbook of Liquid Crystal Research edited by P. J. Collings, J. S. Patel, Ozford University Press Co. (1997, New York), Chapter 2: Chiral and Achiral Calamitic Liquid Crystals for Display Application, A. W. Hall, J. Hollingshurst, J. W. Goodby, pp 47–51), its current molecular design is in a trial and error state and no antiferroelectric liquid crystal that can serve for practical purposes has as yet been synthesized. In fact, during the period of fourteen years that has passed since the discovery of an antiferroelectric crystal, more than 100 types of antiferroelectric liquid crystal have been discovered but any one of them is far in properties from being put to practical use.